I uploaded the photos in parallax slider, but they are little blurred. The size of photos is correct, although the size is not relevant: Photos are always blurred. I tried upload photos in higher quality – effect was the same.

I think you are referring to the overlay above the images in the slider and it could be disabled through custom css, you just need to insert this selector into custom css in the Theme Options > styling and click save changes there:
.sequence-canvas .parallax-overlay {
background-image: none !important;
}